Resistive-switching memory elements having improved switching characteristics

ABSTRACT

Resistive-switching memory elements having improved switching characteristics are described, including a memory element having a first electrode and a second electrode, a switching layer between the first electrode and the second electrode comprising hafnium oxide and having a first thickness, and a coupling layer between the switching layer and the second electrode, the coupling layer comprising a material including metal titanium and having a second thickness that is less than 25 percent of the first thickness.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation and claims the benefit of U.S. patentapplication Ser. No 13/656,908, filed Oct. 22, 2012, which is acontinuation of U.S. patent application Ser. No. 12/705,474, filed Feb.12, 2010, issued as U.S. Pat. No. 8,343,813 on Jan. 1, 2013, which is acontinuation-in-part of U.S. patent application Ser. No. 12/608,934filed Oct. 29, 2009, issued as U.S. Pat. No. 8,183,553 on May 22, 2012,all of which are incorporated herein by reference in their entirety.

This application is also a continuation-in-part and claims the benefitof U.S. patent application Ser. No. 13/561,253, filed Jul. 30, 1012,which is a continuation of U.S. patent Ser. No. 13/098,680, filed May 2,2011, issued as U.S. Pat. No. 8,294,242 on Oct. 23, 2012, which is adivision of U.S. patent Ser. No. 12/463,174, filed on May 8, 2009,issued as U.S. Pat. No. 7,960,216 on Jun. 14, 2011, which claims benefitof U.S. Provisional Patent Application No. 61/052,173, filed May 10,2008, all of which are hereby incorporated by reference in theirentirety.

This application is also a continuation-in-part and claims the benefitof U.S. patent application Ser. No. 13/371,220, filed Feb. 10, 1012,which is a continuation of U.S. patent Ser. No. 12/400,655, filed Mar.9, 2009, issued as U.S. Pat. No. 8,143,092 on Mar. 27, 2012, whichclaims benefit of U.S. Provisional Patent Application No. 61/035,354,filed Mar. 10, 2009, all of which are hereby incorporated by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor memories. Morespecifically, resistive-switching memory elements having improvedswitching characteristics are described.

BACKGROUND OF THE INVENTION

Non-volatile memories are semiconductor memories that retain theircontents when unpowered. Non-volatile memories are used for storage inelectronic devices such as digital cameras, cellular telephones, andmusic players, as well as in general computer systems, embedded systemsand other electronic devices that require persistent storage.Non-volatile semiconductor memories can take the form of removable andportable memory cards or other memory modules, can be integrated intoother types of circuits or devices, or can take any other desired form.Non-volatile semiconductor memories are becoming more prevalent becauseof their advantages of having small size and persistence, having nomoving parts, and requiring little power to operate.

Flash memory is a common type of non-volatile memory used in a varietyof devices. Flash memory uses an architecture that can result in longaccess, erase, and write times. The operational speeds of electronicdevices and storage demands of users are rapidly increasing. Flashmemory is proving, in many instances, to be inadequate for non-volatilememory needs. Additionally, volatile memories (such as random accessmemory (RAM)) can potentially be replaced by non-volatile memories ifthe speeds of non-volatile memories are increased to meet therequirements for RAM and other applications currently using volatilememories.

Resistive-switching memories are memories that include aresistive-switching material (e.g. a metal oxide) that changes from afirst resistivity to a second resistivity upon the application of a setvoltage, and from the second resistivity back to the first resistivityupon the application of a reset voltage. Existing resistive-switchingmemories have switching characteristics (e.g. set, reset, and formingvoltages, retention) that are unsuitable for some applications.

Thus, what is needed is a resistive-switching memory element withimproved switching characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings:

FIG. 1 illustrates a memory array of resistive switching memoryelements;

FIGS. 2A and 2B illustrate a memory element including aresistive-switching material and a select element;

FIGS. 3 and 4 are band diagrams and that illustrate energy levels in amemory element with (FIG. 3) and without (FIG. 4) an interface layer;

FIG. 5 is a graph illustrating the dependency of forming voltage on thepresence of an interface layer;

FIG. 6 illustrates a memory element that shares an electrode with adiode that is used as a select element;

FIG. 7 illustrates a portion of a three-dimensional memory array usingmemory elements described herein;

FIGS. 8A and 8B illustrate the memory element and the creation andmanipulation of oxygen vacancies (defects) within the memory elementusing an interface layer;

FIG. 9 is a logarithm of current (I) versus voltage (V) plot for amemory element;

FIG. 10 is a current (I) versus voltage (V) plot for a memory elementthat demonstrates a resistance state change;

FIG. 11 is a flowchart describing a process for controlling theformation of interface layers; and

FIG. 12 is a flowchart describing a process for forming a memory elementaccording to various embodiments.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is provided belowalong with accompanying figures. The detailed description is provided inconnection with such embodiments, but is not limited to any particularexample. The scope is limited only by the claims and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided for thepurpose of example and the described techniques may be practicedaccording to the claims without some or all of these specific details.For the purpose of clarity, technical material that is known in thetechnical fields related to the embodiments has not been described indetail to avoid unnecessarily obscuring the description.

According to various embodiments, resistive-switching memory elementsare described herein. The memory elements generally have ametal-insulator-metal (MIM) structure in which resistive-switchinginsulating layers are surrounded by two conductive electrodes. Someembodiments described herein are memory elements that include electrodesof different materials (e.g. one electrode is doped silicon and one istitanium nitride) surrounding a resistive-switching layer of a metaloxide (e.g. hafnium oxide (HfO₂), thickness ˜20-100 Å) and a couplinglayer that is substantially thinner than the resistive-switching layer(e.g. less than 25% the thickness of the resistive-switching layer). Insome embodiments, the coupling layer can be a metallic material such astitanium. Memory elements including the coupling layer have exhibitedimproved switching characteristics (e.g. lower set, reset, and formingvoltages, and better retention). In some embodiments, theresistive-switching layer includes a higher bandgap material (i.e. amaterial having a bandgap greater than 4 eV such as HfOx, AlOx, TaOx,YOx, ZrOx, CeOx, etc.), however other resistive-switching layers mayinclude materials having a bandgap less than 4 eV (e.g. TiOx).

In other embodiments, a metal-rich metal oxide switching layer andtechniques for forming the metal-rich switching layer are described. Themetal-rich switching layer includes increased numbers of defects (e.g.oxygen vacancies), which can be manipulated to improve switchingcharacteristics. The metal-rich switching layer can be deposited, forexample, by reducing the degree of oxidation that is occurs in an atomiclayer deposition (ALD) or plasma-enhanced ALD (PEALD) process. Inanother embodiment, the coupling layer is a metallic layer (e.g.metallic titanium) that, when the memory is annealed, attracts oxygenfrom the adjacent metal oxide switching layer. This results in an atleast partially oxidized coupling layer (e.g. the coupling layer becomesat least partially titanium oxide) and a switching layer that is metalrich. In further embodiments, techniques for removing or controlling thesize of an interface layer between an electrode and a switching layerdeposited thereon are described.

I. Switching Operation

It is believed that the resistive switching of the memory elementsdescribed herein is caused by defects in a metal oxide switching layerof the memory element. Generally, defects are formed in or already existin the deposited metal oxide, and existing defects can be enhanced byadditional processes. For example, physical vapor deposition (PVD)processes and atomic layer deposition (ALD) processes deposit layersthat can have some imperfections or flaws. Defects may take the form ofvariances in charge in the structure of the metal oxide: some chargecarriers may be absent from the structure (i.e. vacancies), additionalcharge carriers may be present (i.e. interstitials), or one element cansubstitute for another (i.e. substitutional).

The defects are thought to be electrically active defects (also known astraps) in the bulk of the metal oxide and/or at the interface of themetal oxide and adjoining layers. It is believed that the traps can befilled by the application of a set voltage (to switch from a high to alow resistance state), and emptied by applying a reset voltage (toswitch from the low to the high resistance state). Traps can be inherentin the as-deposited metal oxide (i.e., existing from formation of themetal oxide) or created and/or enhanced by doping and other processes.Doping can be performed using adjacent “doping” layers that interdiffusewith the switching layer, using implantation, or using other techniques.

It is believed that the defects in the switching layer form conductivepercolation paths upon the application of the set voltage. It is furtherbelieved that the percolation paths are removed upon the application ofa reset voltage. For example, a hafnium oxide layer may include oxygenor hafnium vacancies or oxygen or hafnium interstitials that may formtraps which can be used to create percolation paths and alter theconductivity of the hafnium oxide layer.

The switching characteristics of the resistive-switching memory elementscan be tailored by controlling the defects within the metal oxides.Switching characteristics include operating voltages (e.g. set, reset,and forming voltages), operating currents (e.g. on and off currents),and data retention. Defect control is achieved by type, density, energylevel, and spatial distribution within the switching layer. Thesedefects then modulate the current flow based on whether they are filled(passivated/compensated) or unfilled (uncompensated). Adding differentlayers, controlling the formation of the switching layer, implanting,controlling stress, certain thermal treatments are all used to controlthe defect characteristics. In addition, the defects need not be mobile.For example, a coupling layer 212 (see FIG. 2A) and an interface layer214 (see FIGS. 2A and 8A-8B) can be used to control locations, depths,densities, and/or type of defects, and techniques can be used to form aswitching layer having an increased number of defects.

Additionally, the switching layer can have any phase (e.g., crystallineand amorphous) or mixtures of multiple phases. Amorphous-phase metaloxides may have increased resistivity, which in some embodiments canlower the operational currents of the device to reduce potential damageto the memory element.

II. Memory Structure

A. Memory Array

FIG. 1 illustrates a memory array 100 of resistive switching memoryelements 102. Memory array 100 may be part of a memory device or otherintegrated circuit. Memory array 100 is an example of potential memoryconfigurations; it is understood that several other configurations arepossible.

Read and write circuitry may be connected to memory elements 102 usingsignal lines 104 and orthogonal signal lines 106. Signal lines such assignal lines 104 and signal lines 106 are sometimes referred to as wordlines and bit lines and are used to read and write data into theelements 102 of array 100. Individual memory elements 102 or groups ofmemory elements 102 can be addressed using appropriate sets of signallines 104 and 106. Memory element 102 may be formed from one or morelayers 108 of materials, as is described in further detail below. Inaddition, the memory elements 102 shown can be stacked in a verticalfashion to make multi-layer 3-D memory arrays (see FIG. 7).

Any suitable read and write circuitry and array layout scheme may beused to construct a non-volatile memory device from resistive switchingmemory elements such as element 102. For example, horizontal andvertical lines 104 and 106 may be connected directly to the terminals ofresistive switching memory elements 102. This is merely illustrative.

If desired, other electrical devices may be associated (i.e., be one ormore of the layers 108) with each memory element 102 (see, e.g., FIG.2A). These devices, which are sometimes referred to as select elements,may include, for example, diodes, p-i-n diodes, silicon diodes, siliconp-i-n diodes, transistors, Schottky diodes, etc. Select elements may beconnected in series in any suitable locations in memory element 102.

B. Memory Element

1. MIM Structure

FIGS. 2A and 2B illustrate a memory element 102 including aresistive-switching material and a select element (a diode 202). FIG. 2Bis an enlarged view of a portion of the memory element 102 according toone embodiment. The memory element 102 includes a metal-insulator-metal(MIM)-style stack 204 (in some embodiments, one or more of the metallayers can be a conductive semiconductor material such as dopedsilicon). The stack 204 includes two electrodes 206 and 208 and aresistive-switching layer 210 (e.g. an insulator or metal oxide). Theelectrodes 206 and 208 can be metals, metal carbides, metal oxides, ormetal nitrides (e.g. Pt, Ru, RuO₂, Ir, IrO₂, TiN, W, TaN, MoN, MoOx, WN,TiAl alloys, Ni, WOx, Al, doped Al, Cu, HfC, HfN, NbN, TaC, TaSiN, Ti,VC, VN, ZrN, etc.), or can be doped silicon, for example p- or n-typedoped polysilicon. The resistive-switching layer 210 can be a metaloxide or other switching material. In some embodiments, theresistive-switching layer 210 is a high bandgap (i.e. bandgap greaterthan four electron volts (eVs)) material such as HfO₂, Ta₂O₅, Al₂O₃,Y₂O₃, CeO₂ and ZrO₂. In other embodiments, lower bandgap metal oxidematerials can be used, such as or TiO₂. CeO₂ may be advantageous forsome embodiments because it may include ions that have higher mobility.In further embodiments, a semiconductive metal oxide (p-type or n-type)such as ZnO, CuO, and their nonstoichiometric and doped variants can beused because it is believed that these oxides may have advantageousswitching characteristics.

a. Switching Layer

The switching layer 210 can have any desired thickness, but in someembodiments can be between 10 and 100 Å, between 20 and 60 Å, orapproximately 50 Å. The switching layer 210 can be deposited using anydesired technique, but in some embodiments described herein is depositedusing ALD. In other embodiments, the switching layer 210 can bedeposited using low pressure CVD (LPCVD), plasma enhanced CVD (PECVD),plasma enhanced ALD (PEALD), physical vapor deposition (PVD), liquiddeposition processes, and epitaxy processes. It is believed that PEALDprocesses can be used to control defects and improve switching andforming voltages in some embodiments.

The switching layer 210 additionally can be metal-rich (e.g. HfO_(1.7)vs. HfO₂) such that the elemental composition of the switching layer 210is less than stoichiometric (e.g. less than HfO₂). The switching layer210 can have a deficit of oxygen, which manifests as oxygen vacancydefects. The additional defects can lead to reduced and more predictableswitching and forming voltages of the memory element 102. Techniques fordepositing a metal-rich switching layer 210 are described in FIG. 11.

In some embodiments, higher bandgap (i.e. greater than 4 eV) metaloxides have set voltages that increase with increased thickness (i.e.are scalable), which indicates a bulk-switching property and which maybe desirable for the ability to alter operating voltages based on layerthickness. Examples of these materials include those shown in Table 1.In other embodiments, ternary metal oxides, such as Hf_(x)M_(y)O_(z),where M=Al, Zr, Ti, La, or Sr can also be used for the switching layer210.

TABLE 1 Material Bandgap HfO2 5.7 eV Al2O3 8.4 eV Ta2O5 4.6 eV Y2O3 6.0eV ZrO2 5.8 eV CeO2 5.5 eVb. Coupling Layer

The stack 204 can also include a coupling layer 212, which may beanother metal oxide such as ZrO₂ or Al₂O₃, or a metallic layer, such asmetallic Ti. In other embodiments, the coupling layer 212 can bedeposited as a metal layer that will partially or fully oxidize upon thedeposition of the adjacent electrode 208 or upon annealing. The couplinglayer 212 can, for example, facilitate switching at the electrode 208 bycreating defects near the electrode 208. For example, if the couplinglayer is a metal such as Ti, during an anneal (such as rapid thermaloxidation or a vacuum anneal), the coupling layer 212 can attract oxygenfrom the adjacent metal oxide switching layer 210, thereby creatingoxygen vacancies in the switching layer 210 while oxidizing the couplinglayer 212.

For example, as shown in FIG. 2B, a memory element 102 includes acoupling layer 212 that is substantially metallic titanium (or anothermetal) as deposited, and a switching layer that is a metal oxide (e.g.HfO₂). After the memory element 102 is annealed or as a result of thedeposition of the electrode 208, the metallic coupling layer 212 canattract oxygen from the switching layer 210, resulting in the couplinglayer 212 and switching layer 210 having multiple portions havingdifferent characteristics (e.g. different oxygen concentrations). Forexample, a first portion 212 a of the coupling layer 212 issubstantially metallic (e.g. substantially metallic titanium), while asecond portion 212 b that is in contact with the switching layer 210 issubstantially metal oxide (e.g. titanium oxide). At the same time, afirst portion 210 a of the switching layer 210 that is in contact withthe coupling layer 210 is oxygen deficient because the oxygen in themetal oxide has migrated to the second portion 212 b of the couplinglayer 212, leaving oxygen vacancies which can be used to tailorswitching characteristics. A second portion 210 b of the switching layer210 has an oxygen concentration that is greater than that of the firstportion 210 b. In other words, the first portion 210 a is more metalrich (e.g. HfO_(1.2-1.5)) than the second portion (e.g. HfO_(1.6-2.0)).It is understood that the portions 210 a, 210 b, 212 a, and 212 b aremerely illustrative, and that each of the switching layer 210 and thecoupling layer 212 may have several portions having differentconcentrations of oxygen, or that the oxygen concentration may begradated through the thickness of the layers 210 and 212. Further, theamount of oxidation of the coupling layer 212 and the reduction ofoxygen of the switching layer 210 can be tailored by changing theparameters (e.g. duration, oxygen concentration for RTO) of the anneal.

The coupling layer 212 can be thinner than the resistive-switching layer210, for example the coupling layer 212 can have a thickness that isless than 25% of the thickness of the resistive-switching layer 210, ora thickness that is less than 10% of the thickness of theresistive-switching layer 210. For example, the resistive-switchinglayer 210 can be a 20-60 Å layer, and the interface layer 212 can be a5-10 Å layer. The coupling layer 212 is thin enough to provide access todefects in the switching layer 210.

In some embodiments, the coupling layer 212 is metallic titanium asdeposited, which in some embodiments can become at least partiallytitanium oxide during an anneal. A titanium coupling layer 212 canchange the effective work function of the adjacent electrode 208, whichcan be used to modify switching characteristics (e.g. by reducingleakage).

c. Interface Layer

The stack 204 further may include an interface layer 214 between theelectrode 206 and the switching layer 210. The interface layer 214 canbe an oxide of the material of the electrode 206 that is formed as aresult of and during the deposition of the switching layer 210, forexample as a result of thermal oxidation during processing. Theinterface layer 214 can, in some embodiments, alter defects in theswitching layer 210 (see, e.g. FIGS. 8A-8B). In other embodiments, itmay be desirable to eliminate the interface layer 214 to reduce formingvoltage or to enable switching. It is believed that in some embodiments,the interface layer 214 can hinder effective electron injection into theswitching layer 210 that enables traps to be filled, which therebyincreases forming voltage or causes excessive potential drop across theswitching layer 210, producing high electric fields in the switchinglayer 210 and preventing switching. Techniques for controlling the sizeof or eliminating the interface layer 214 are described in FIGS. 11 and12.

FIGS. 3 and 4 are band diagrams 300 and 400 that illustrate energylevels in a memory element with (FIG. 3) and without (FIG. 4) aninterface layer 214. For each of the band diagrams 300 and 400, thereare corresponding electric field diagrams 320 and 340 that illustratethe strength of the electric field within a certain region of the memoryelement 102.

In the band diagram 300, a memory element has a titanium nitrideelectrode 302, a zirconium oxide coupling layer 304, a hafnium oxideswitching layer 306, a silicon oxide interface layer 308, and an n-typepolysilicon electrode 310. The materials used here are illustrations; itis understood that other materials can also be used (e.g. metallictitanium instead of zirconium oxide). In the band diagram 400, a memoryelement has a titanium nitride electrode 402, a zirconium oxide couplinglayer 404, a hafnium oxide switching layer 406, and an n-typepolysilicon electrode 408. As is shown in the electric field diagram320, the electric field is reduced by a large amount 322 in theinterface layer 314. Increased switching voltages may be necessary toovercome the electric field reduction in the interface layer 214. If theinterface layer 214 is thick enough, the entire electric field may belost to the interface layer 214, which may prevent switching altogether.Alternatively, as is shown in the electric field diagram 420, in thememory element without the interface layer 214 the electric field isreduced evenly 422 throughout the memory element 102, including in theswitching layer 210, which can reduce switching voltages and lead tomore predictable switching. However, as is described regarding FIGS. 8Aand 8B, it may be desirable to retain a controlled-thickness interfacelayer 214 to increase the number of defects in the switching layer 210.

FIG. 5 is a graph 500 illustrating the dependency of forming voltage onthe presence of an interface layer 214. Three sets of memory elementswere prepared:

A first set of memory elements represented by diamonds 502 includes atitanium nitride electrode 206, a PVD-deposited hafnium oxide switchinglayer 210, and a platinum electrode 208 without a coupling layer 212.

A second set of memory elements represented by squares 504 includes ann-type polysilicon electrode 206, an ALD-deposited hafnium oxideswitching layer 210, and a titanium nitride electrode 208 without acoupling layer 212.

A third set of memory elements represented by a circle 506 includes ann-type polysilicon electrode 206, a PVD-deposited hafnium oxideswitching layer 210, and a platinum electrode 208 without a couplinglayer 212.

The graph 500 shows the median forming voltage of the memory elements asa function of the thickness of the switching layer in the memoryelements. As can be seen, for a switching layer having the samethickness, the elements 502 including PVD hafnium oxide on titaniumnitride have the lowest forming voltage, elements 506 including PVDhafnium oxide on polysilicon have the next lowest forming voltage, andelements 504 having ALD hafnium oxide on polysilicon have the highestforming voltage. It is believed that ALD processes are more likely toform a thicker interface layer 214 at least partly because ofpotentially higher processing temperatures (200° C. or greater versusroom temperature for some instances of PVD), which leads to increasedforming voltages. Additionally, the silicon oxide interface layer 214created on polysilicon electrodes (e.g. the elements 502 and 506) isless conductive than an oxide created on a metal-containing electrodesuch as titanium nitride. Therefore, techniques for reducing and/orcontrolling the interface layer 214, especially for silicon-basedelectrodes, can be used to improve forming voltages.

Although ALD process may be more likely to form thicker interface layers214 and result in memory elements having increased forming voltages, itmay be desirable to use ALD processing over PVD processing for otherreasons (e.g. to form more conformal layers), and FIG. 11 describes aprocess for reducing or eliminating the interface layer 214 using ALDprocessing. Additionally, as is described regarding FIGS. 8A and 8B, itmay be desirable to retain a controlled-thickness interface layer 214(e.g. less than or equal to 10 Å) to increase the number of defects inthe switching layer 210, which can also be formed using the process ofFIG. 11.

If it is desirable to have an interface layer 214, the order ofdeposition of the layers of the MIM stack 204 may be important. Sincethe interface layer 214 is formed during the deposition of the switchinglayer 210, the switching layer 210 can be formed on the electrode thatthe interface layer 214 is to be formed from (e.g. formed on thepolysilicon layer if a silicon oxide interface layer 214 is desired). Asan example, and as is discussed further in FIG. 7, when forming athree-dimensional memory array, it may be necessary to always form thememory element in the same orientation (e.g. one electrode always on thebottom), even when the orientation of other elements is to be reversed.In other embodiments however, the interface layer 214 can be createdwhen the memory element 102 is deposited in reverse order by using apost deposition anneal of the memory element 102.

d. Electrodes

The electrodes 206 and 208 can be different materials. In someembodiments, the electrodes have a work function that differs by between0.1 and 1 electron volt (eV), or by between 0.4 and 0.6 eV, etc. Forexample, the electrode 208 can be TiN, which has a work function of4.5-4.6 eV, while the electrode 206 can be n-type polysilicon, which hasa work function of approximately 4.1-4.15 eV. Other electrode materialsinclude p-type polysilicon (4.9-5.3 eV), transition metals, transitionmetal alloys, transition metal nitrides, transition metal carbides,tungsten (4.5-4.6 eV), tantalum nitride (4.7-4.8 eV), molybdenum oxide(approximately 5.1 eV), molybdenum nitride (4.0-5.0eV), iridium (4.6-5.3eV), iridium oxide (approximately 4.2 eV), ruthenium (approximately 4.7eV), and ruthenium oxide (approximately 5.0 eV). Other potentialelectrodes include a titanium/aluminum alloy (4.1-4.3 eV), nickel(approximately 5.0 eV), tungsten nitride (approximately 4.3-5.0 eV),tungsten oxide (5.5-5.7 eV), aluminum (4.2-4.3 eV), copper orsilicon-doped aluminum (4.1-4.4 eV), copper (approximately 4.5 eV),hafnium carbide (4.8-4.9 eV), hafnium nitride (4.7-4.8 eV), niobiumnitride (approximately 4.95 eV), tantalum carbide (approximately 5.1eV), tantalum silicon nitride (approximately 4.4 eV), titanium (4.1-4.4eV), vanadium carbide (approximately 5.15 eV), vanadium nitride(approximately 5.15 eV), and zirconium nitride (approximately 4.6 eV).For some embodiments described herein, the higher work functionelectrode receives a positive pulse (as measured compared to a commonreference potential) during a reset operation, although otherconfigurations are possible. In other embodiments, the higher workfunction electrode receives a negative pulse during a reset operation.In some embodiments, the memory elements 102 use bipolar switching wherethe set and reset voltages have opposite polarities relative to a commonelectrical reference, and in some embodiments the memory elements 102use unipolar switching where the set and reset voltages have the samepolarity. In other embodiments, it is desirable to generally increasethe work function of the electrodes in order to reduce leakage andcreate a more stable metal oxide/electrode interface.

2. Select Elements

The diode 202 is a select element that can be used to select a memoryelement for access from amongst several memory elements such as theseveral memory elements 102 of the memory array 100 (see FIG. 1). Thediode 202 controls the flow of current so that current only flows oneway through the memory elements 102.

The diode 202 may include two or more layers of semiconductor material.A diode is generally a p-n junction, and doped silicon layers 216 and218 can form the p-n junction. For example, doped silicon layer 216 canbe a p-type layer and doped silicon layer 218 can be an n-type layer, sothat a node 220 of the diode 202 is an anode and is connected to thefirst electrode 206. In this example, a node 222 of the diode 202 is acathode and is connected to the signal line 106, which may be, forexample, a bit line or word line, or connected to a bit line or wordline. The nodes 220 and 222 are not necessarily physical features in thememory element 102, for example the electrode 206 may be in directcontact with the doped silicon layer 216. In other embodiments, one ormore additional layers such as a low resistivity film are added betweenthe electrode 206 and the doped silicon layer 216.

In some embodiments, doped silicon layer 216 is an n-type layer anddoped silicon layer 218 is a p-type layer, and the node 220 is a cathodeof the diode 202 and the node 222 is an anode of the diode 202. Anoptional insulating layer 224 can be between the doped silicon layers216 and 218 to create a p-i-n or n-i-p diode 202. In some embodimentsthe insulating layer 224 and one of the doped silicon layers 216 and 218are formed from the same layer. For example, a silicon layer can bedeposited, and a portion of the layer can be doped to form the dopedsilicon layer 216 or 218. The remaining portion of the layer is then theinsulating layer 224.

In other embodiments, one electrode of the memory element 102 can bedoped silicon (e.g. p-type or n-type polysilicon), which can also act asa portion of the diode 202. FIG. 6 illustrates a memory element 102 thatshares an electrode with a diode 202 that is used as a select element.Since the diode 202 is made up of two layers of doped silicon, and sincea layer of doped silicon can be used as an electrode of the memoryelement 202, a single layer of doped silicon (e.g. a layer of n-typepolysilicon) can serve as an electrode of the memory element 102 and asa layer of the diode 202. By sharing a doped silicon layer between thediode 202 and the memory element 102, two layers, one doped siliconlayer and a coupling layer between the diode 202 and the memory element102, and their associated processing steps, can be eliminated.

3. Switching Polarity

A signal line (e.g. the signal line 104) is connected to the “second”electrode 208, and the signal line is configured to provide switchingvoltages to the second electrode 208. In some embodiments, the secondelectrode 208 has a higher work function than the first electrode 206,and the signal line 104 is configured to provide a negative set voltagerelative to a common electrical reference, and a positive reset voltagerelative to the common electrical reference. The embodiments may includethose using a lower work function first electrode 206 (e.g. titaniumnitride) and a higher work function second electrode such as platinum orruthenium. For example, the common electrical reference may be ground(i.e. 0V), the set voltage would then be a negative voltage (e.g. −2V),and the reset voltage would be a positive voltage (e.g. 2V). The commonelectrical reference can be any voltage, however, such as +2V or −2V.

In other embodiments, the second electrode 208 also has a higher workfunction than the first electrode 206, and the signal line 104 isconfigured to provide a positive set voltage and a negative resetvoltage relative to a common electrical reference. For example, in amemory element having a doped silicon first electrode 206 (e.g. n-typepolysilicon) and a higher work function second electrode 208 (e.g.titanium nitride), the reset voltage can be negative at the secondelectrode 208.

In some embodiments, one switching voltage (e.g. the reset voltage) ofthe memory element can have a first polarity (e.g. a positive polarity)relative to the common electrical reference, and the other switchingvoltage (e.g. the set voltage) can have a negative polarity relative tothe common electrical reference so that the memory element uses bipolarswitching. In other embodiments, the switching voltages have the samepolarity relative to a common reference and are referred to as unipolarswitching. In some embodiments, it may be desirable to use unipolarswitching to provide compatibility with certain configurations of otherelements of the memory array (e.g. select elements). Additionally, theswitching voltages can be voltage pulses (e.g. square wave pulses)having a limited duration, for example less than 1 ms, less than 50 μs,less than 1 μs, less than 50 ns, etc.

In one embodiment, the switching layer 210 can be a metal alloy, such asa Ni/Ti alloy. It is believed that the types of bonds in the alloy (i.e.covalent v. ionic) can influence whether the switching is unipolar orbipolar. In other embodiments, the switching layer 210 can be p-typemetal oxides, such as doped ZnO or doped CuO.

4. Other Characteristics

It may be desirable to have a low-leakage material as theresistive-switching layer 210 in order to aid memory retention. Forexample, the layer 210 may be a material that has a leakage currentdensity less than 40 amps per square centimeter (A/cm²) measured at 0.5volts (V) per twenty angstroms of the thickness of the metal oxide in anoff state (e.g. a high resistance state) of the memory element.

5. 3-D Memory Structure

FIG. 7 illustrates a portion of a three-dimensional memory 700 arrayusing memory elements 102 described herein. The array 700 includes twoword lines 702 a and 702 b, and a shared bit line 704. Two MIM stacks204 a and 204 b and diodes 202 a and 204 b are shown in the array 700; amemory cell 706 a includes an MIM stack 204 a and a diode 202 a, and amemory cell 706 b includes an MIM stack 204 b and a diode 202 b.

The memory array 700 is configured so that the two memory cells 706 aand 706 b can use the same shared bit line 704. As shown here, the MIMstacks 204 a and 204 b both have their individual layers (i.e.electrodes 206 and 208 and switching layer 210) built in the same order.In other words, for both MIM stacks 204 a and 204 b, the electrode 206is formed first, the switching layer 210 is formed on top of theelectrode 206, and the electrode 208 is formed on top of the switchinglayer 210. As mentioned above, the order of deposition of the layers ofthe MIM stacks 204 may need to be the same in order to create aninterface layer 214. However, in some embodiments the order ofdeposition can be reversed and the interface layer 214 created as aresult of subsequent processes such as electrode deposition orannealing.

The diodes 202 a and 202 b, on the other hand, are mirrors of eachother. In other words the diode 202 a has the layer 216 on the bottom,and the diode 202 b has the layer 218 on the bottom. For example, thelayer 216 may be the n-type layer and the layer 218 may be the p-typelayer. Using this configuration, the diodes 202 a and 202 b are biasedin opposite directions, which allows the memory cells 706 to both usethe same shared bit line 704. As is shown in circuit diagrams 708 a and708 b, the diodes can have any desired orientation, and the orientationmay differ based on the configuration of the three-dimensional memoryarray.

6. Interface Layer and Oxygen Vacancies

FIGS. 8A and 8B illustrate the memory element 102 and the creation andmanipulation of oxygen vacancies (defects) within the memory element 102using an interface layer 214. The interface layer 214 is an oxide layerthat can be created during the processing of other layers in the memoryelement 102. For example, the deposition of the switching layer 210 mayinclude processing at a temperature (e.g. 200° C. or greater) to createthe interface layer 214. If, for example, the electrode 206 is dopedsilicon (e.g. polysilicon), the deposition of the switching layer 210(using, for example, PVD or ALD) may include temperatures that cancreate a silicon oxide interface layer 214. The interface layer 214 canbe eliminated in some embodiments, but in other embodiments, theinterface layer 214 can be retained to improve retention of theswitching layer 210 by improving leakage characteristics and to modulatedefects (e.g. oxygen vacancies) in the switching layer 210. In someembodiments where the interface layer 214 is retained, the interfacelayer 214 may be relatively thin (e.g. less than or equal to 10 Å) tomake the defects in the switching layer 210 visible to the electrode 206(i.e. the interface layer 214 provides access to the defects of theswitching layer 210) and to reduce the effect of the interface layer 214on switching voltages.

In one example, the bottom electrode 206 is polysilicon. Silicon,particularly, is known for attracting oxygen when heated and can drawoxygen from the metal oxide switching layer 210, leaving oxygenvacancies 802 in the switching layer 210 nearby creating a metal-richmetal oxide switching layer. Without being bound by theory, these oxygenvacancies 802 can serve as traps which modulate the current flow withthe application of programming voltages to fill and empty such traps.The oxygen vacancies 802 need not be mobile. In some embodiments, theexistence of an interface layer 214 can be combined with the existenceof a metallic coupling layer 212 that also attracts oxygen, which can,depending on the processing conditions, create additional oxygenvacancies on both sides of the switching layer 210 or throughout theswitching layer 210.

A thin or zero interlayer thickness interface layer 214 can be used tomodulate the density of oxygen vacancies 802 in the switching layer 210.For example, a thinner interface layer 214 (e.g. 5 Å vs. 10 Å) canincrease the oxygen vacancy 802 density. Additionally, the thickness ofthe switching layer 210 can be optimized such that traps (e.g. oxygenvacancies 802) are more spatially equalized throughout the switchinglayer 210. For example, FIG. 8A shows a thicker switching layer 210,which has oxygen vacancies 802 concentrated near the interface layer214, while FIG. 8B shows a thinner switching layer 210 that has a moreeven distribution of oxygen vacancies 802. For example, in two memoryelements using the same materials, the switching layer 210 of FIG. 8Amay be 50 Å while the thickness of the switching layer 210 in FIG. 8B is25 Å. The distribution of oxygen vacancies 802 within the switchinglayer 210 can depend on several factors, including the materials used,the thickness of the interface layer 214, the processes used (e.g.temperatures of anneals used), etc. FIGS. 8A and 8B are only twoexamples of oxygen vacancy distribution, and it is understood thatvarious other configurations are possible.

III. Memory Operation

During a read operation, the state of a memory element 102 can be sensedby applying a sensing voltage (i.e., a “read” voltage V_(READ)) to anappropriate set of signal lines 104 and 106. Depending on its history, amemory element that is addressed in this way may be in either a highresistance state or a low resistance state. The resistance of the memoryelement therefore determines what digital data is being stored by thememory element. If the memory element has a low resistance, for example,the memory element may be said to contain a logic one (i.e., a “1” bit).If, on the other hand, the memory element has a high resistance, thememory element may be said to contain a logic zero (i.e., a “0” bit).During a write operation, the state of a memory element can be changedby application of suitable write signals to an appropriate set of signallines 104 and 106.

FIG. 9 is a logarithm of current (I) versus voltage (V) plot 900 for amemory element 102. FIG. 9 illustrates the set and reset operations tochange the contents of the memory element 102. Initially, memory element102 may be in a high resistance state (“HRS”, e.g., storing a logiczero). In this state, the current versus voltage characteristic ofmemory element 102 is represented by solid line HRS 902. The highresistance state of memory element 102 can be sensed by read and writecircuitry using signal lines 104 and 106. For example, read and writecircuitry may apply a read voltage V_(READ) to memory element 102 andcan sense the resulting “off” current I_(OFF) that flows through memoryelement 102. When it is desired to store a logic one in memory element102, memory element 102 can be placed into its low-resistance state.This may be accomplished by using read and write circuitry to apply aset voltage V_(SET) across signal lines 104 and 106. Applying V_(SET) tomemory element 102 causes memory element 102 to switch to its lowresistance state, as indicated by dashed line 906. In this region, thememory element 102 is changed so that, following removal of the setvoltage V_(SET), memory element 102 is characterized by low resistancecurve LRS 904. As is described further below, the change in theresistive state of memory element 102 may be because of the filling oftraps (i.e., a may be “trap-mediated”) in a metal oxide material.V_(SET) and V_(RESET) can be generally referred to as “switchingvoltages.”

The low resistance state of memory element 102 can be sensed using readand write circuitry. When a read voltage V_(READ) is applied toresistive switching memory element 102, read and write circuitry willsense the relatively high “on” current value I_(ON), indicating thatmemory element 102 is in its low resistance state. When it is desired tostore a logic zero in memory element 102, the memory element can onceagain be placed in its high resistance state by applying a reset voltageV_(RESET) to memory element 102. When read and write circuitry appliesV_(RESET) to memory element 102, memory element 102 enters its highresistance state HRS, as indicated by dashed line 908. When the resetvoltage V_(RESET) is removed from memory element 102, memory element 102will once again be characterized by high resistance line HRS 904.Voltage pulses can be used in the programming of the memory element 102.For example, a 1 ms, 10 μs, 5 μs, 500 ns, etc. square pulse can be usedto switch the memory element 102; in some embodiments, it may bedesirable to adjust the length of the pulse depending on the amount oftime needed to switch the memory element 102.

A forming voltage V_(FORM) is a voltage applied to the memory element102 to ready the memory element 102 for use. Some memory elementsdescribed herein may need a forming event that includes the applicationof a voltage greater than or equal to the set voltage or reset voltage.Once the memory element 102 initially switches the set and resetvoltages can be used to change the resistance state of the memoryelement 102.

The bistable resistance of resistive switching memory element 102 makesmemory element 102 suitable for storing digital data. Because no changestake place in the stored data in the absence of application of thevoltages V_(SET) and V_(RESET), memory formed from elements such aselement 102 is non-volatile.

FIG. 10 is a current (I) versus voltage (V) plot 1000 for a memoryelement 102 that demonstrates a resistance state change. The plot 1000shows a voltage ramp applied to the memory element 102 along the x-axisand the resulting current along a y-axis. The line 1002 represents theresponse of an Ohmic material when the ramped voltage is applied. AnOhmic response is undesirable, since there is no discrete voltage atwhich the set or reset occurs.

Generally, a more abrupt response like graph 1004 is desired. The graph1004 begins with an Ohmic response 1004 a, and then curves sharplyupward 1004 b. The graph 1004 may represent a set operation, where thememory element 102 switches from the HRS 902 to the LRS 904.

Without being bound by theory, non-metallic percolation paths are formedduring a set operation and broken during a reset operation. For example,during a set operation, the memory element 102 switches to a lowresistance state. The percolation paths that are formed by filling trapsincrease the conductivity of the metal oxide, thereby reducing (i.e.,changing) the resistivity. The voltage represented by 404 b is the setvoltage. At the set voltage, the traps are filled and there is a largejump in current as the resistivity of the metal oxide decreases.

IV. Materials

A variety of metal oxides can be used for the switching layer 210 of thememory elements 102 described herein. In some embodiments, the memoryelements 102 exhibit bulk-switching properties and are scalable. Inother words, it is believed that defects are distributed throughout thebulk of the switching layer 210, and that the switching voltages (i.e.V_(SET) and V_(RESET)) increase or decrease with increases or decreasesin thickness of the metal oxide. In other embodiments, the memoryelements 102 exhibit interface-mediated switching activity. Otherembodiments may exhibit a combination of bulk- and interface-mediatedswitching properties, which may be scalable while still exhibitingdefect activity at layer interfaces.

Table 1 includes a list of possible materials systems for memoryelements 102 described herein. Although certain combinations aredescribed in Table 1, various other configurations are possible withinthe bounds of the memory elements 102 described herein. For example,other electrode materials or switching materials can be used.

TABLE 1 Interface Layer Switching Coupling Layer Electrode 206 214 Layer210 212 Electrode 208 1 n-type polysilicon 0-10A SiOx HfOx 30-100A TiOx(1-15A or TiN, TaN, W, p-type polysilicon or ~50A 5A or 8A), AlOx WN,Ni, Cu, Al, (1-15A or 5A or TiAl 8A), ZrOx (1-15A or 5A or 8A) or None 2n-type polysilicon 0-10A SiOx HfxMyOz 30-100A TiOx (1-15A or TiN, TaN,W, p-type polysilicon or ~50A 5A or 8A), AlOx WN, Ni, Cu, Al, where M =Al, (1-15A or 5A or TiAl Zr, Ti, La, Sr 8A), ZrOx (1-15A or 5A or 8A) orNone 3 n-type polysilicon 0-10A SiOx HfOx, CeOx, Ti, TiOx or TiN, TaN,W, p-type polysilicon AlOx, TaOx, None WN, Ni, Cu, Al, Yox, ZrOx, TiAlZnOx, CuOx 4 TiN, TaN, W, WN, None HfOx 30-100A Ti, TiOx or TiN, TaN, W,Ni, Cu, Al, TiAl or ~50A None WN, Ni, Cu, Al, TiAl 5 TiN, TaN, W, WN,None HfxMyOz 30-100A Ti, TiOx or TiN, TaN, W, Ni, Cu, Al, TiAl or ~50ANone WN, Ni, Cu, Al, where M = Al, TiAl Zr, Ti, La, Sr 6 TiN, TaN, W,WN, None HfOx, CeOx, Ti, TiOx or TiN, TaN, W, Ni, Cu, Al, TiAl AlOx,TaOx, None WN, Ni, Cu, Al, Yox, ZrOx, TiAl ZnOx, CuOxV. Processing

FIG. 11 is a flowchart describing a process 1100 for controlleddeposition of an interface layer 214. The process 1100 describes thedeposition of a switching layer 210 using an ALD process that reducesthe amount of oxygen introduced to create a metal-rich switching layer210 and increase the amount of defects in the switching layer 210.Additionally, the process 1100 can be used to tailor the size of theinterface layer 214 by selecting processing parameters to obtain adesired thickness of the interface layer 214.

Atomic layer deposition (ALD) is a process used to deposit conformallayers with atomic scale thickness control during various semiconductorprocessing operations. For depositing a metal oxide, ALD is a multi-stepself-limiting process that includes the use of two reagents: a metalprecursor and an oxygen source (e.g. an oxidant). Generally, a firstreagent is introduced into a processing chamber containing a substrateand adsorbs on the surface of the substrate. Excess first reagent ispurged and/or pumped away. A second reagent is then introduced into thechamber and reacts with the adsorbed layer to form a deposited layer viaa deposition reaction. The deposition reaction is self-limiting in thatthe reaction terminates once the initially adsorbed layer is consumed byreaction with the second reagent. Excess second reagent is purged and/orpumped away. The aforementioned steps constitute one deposition or ALD“cycle.” The process is repeated to form the next layer, with the numberof cycles determining the total deposited film thickness.

Returning to FIG. 11, the process 1100 begins with depositing a bottomelectrode on a substrate in operation 1102. The bottom electrode (e.g.the electrode 206) may be one of the electrode materials describedabove; however, in one embodiment, the bottom electrode is a polysiliconelectrode that may form a silicon dioxide interface layer 214 during thedeposition of the switching layer 210. In other embodiments, the bottomelectrode is a metal electrode that can also oxidize during thedeposition of the switching layer 210.

In operation 1104, a switching layer 210 is deposited using ALD. Theoperation 1104 includes several component operations 1106-1118 thatdescribe several cycles of the ALD process. Some of these operations areoptional, or may be completed in a different order.

In operation 1106, the deposition temperature of the ALD process isoptionally lowered. The deposition temperature may be lowered bylowering the temperature of a heated substrate pedestal (i.e. thepedestal temperature), for example. In some examples, the depositiontemperature or pedestal temperature may be 250° C. or less, 200° C. orless, 175° C. or less, etc. Lower temperatures may change theequilibrium conversion of surface species during oxidation, altering theconcentration of electrical defects in the switching layer 210.Additionally, the reduced deposition temperature can reduce or eliminatethe interface layer 214 by reducing the rate of thermal oxidation. Forexample, when using a silicon electrode 206, reducing the ALD depositiontemperature to below 200° C. may substantially reduce any interfacelayer 214.

In operation 1108, the precursor source is maintained at a desiredpressure. The desired vapor pressure can be achieved by controlling thetemperature of the precursor source. The precursor source is external tothe ALD deposition chamber, and therefore can be maintained at atemperature different than the temperature of the deposition chamber.The desired temperature and pressure depends on the precursor used. Forexample, when using tetrakis(dimethlyamino)hafnium (TDMAH) to deposithafnium oxide, the precursor source can be maintained at 30-100° C., or40-50° C. In some embodiments, the temperature of the precursor sourcecan be increased to increase the partial pressure of the precursor,which can also create a more metal-rich switching layer by increasingthe concentration of metal precursor in the chamber. In operation 1110,the precursor is introduced to the substrate including the bottomelectrode to begin the ALD process.

Operations 1112 and 1114 describe the treatment of the oxygen sourceused to form the metal oxide. Depending on the characteristics of thememory element 102, either or both of operations 1112 and 1114 can beused to control the thickness of the interface layer 214. The oxygensource can be ozone, oxygen, water vapor, isopropyl alcohol (IPA),ethanol or another alcohol, or other ALD oxygen sources. For someconfigurations, using water vapor as an oxygen source in an ALD processhas resulted in memory elements having improved switchingcharacteristics.

In operation 1112, when the oxygen source is a liquid or solid such aswater or isopropyl alcohol, it is maintained at a lower vapor pressurethan is typical to create a switching layer 210 having less oxygen. Thepartial pressure of the oxidant can be reduced by reducing the sourcetemperature which reduces its vapor pressure. The partial pressure canalso be reduced by diluting the oxidant with an inert gas such as argon.Some oxidants such as oxygen and ozone are gases at their sources andtherefore their partial pressure cannot be manipulated by changing thesource temperature. A third method of reducing the degree of oxidationis through selection of the oxidant itself. For example, ozone andoxygen tend to be more oxidizing (i.e. more quickly create a layerhaving more oxygen), while water vapor is less oxidizing, and IPA andethanol are less oxidizing still. Restricting the amount of the oxygensource in the chamber still allows the film to be self-limiting, whilereducing the amount of oxygen in the film. Unreacted ligands attached tometal atoms can be partially or nearly fully removed throughpost-deposition treatments. The oxygen-deficient film will then haveoxygen vacancies, which are defects that can be used to control theswitching of the memory element 102.

To deposit a metal-rich hafnium oxide switching layer 210, for example,water vapor can be used as the oxygen source, and the water vapor sourcecan be held at a reduced temperature such as 0 to 10° C. The reducedtemperature reduces the vapor pressure of the oxygen source, effectivelyreducing the amount of oxidation per ALD cycle. Hafnium oxide filmsformed using this technique can result in elemental compositions ofHfO_(1.2) to HfO_(1.9), or HfO_(1.7). Generally, oxygen concentrationscan be reduced to 60-95% of stoichiometric compositions (i.e. the amountof oxygen is between 60 and 95% of a stoichiometric metal oxide, e.g.HfO_(1.2) to HfO_(1.9)). IPA or ethanol can be used to provide oxygen,but at the same temperature will provide less oxygen than water vapor orthe other oxygen sources described above. IPA or ethanol may thereforebe able to deposit metal-rich films using a room temperature source,although a similar temperature reduction can also be used with IPA andethanol to reduce the amount of oxygen in the switching layer 210.

In operation 1116, the oxygen source is introduced to the substrate tocreate an ALD layer of metal oxide. A single ALD cycle may deposit afilm having a thickness of 0.5 Å, for example, and multiple cycles aretypically needed to build a switching layer 210 of the desiredthickness. In operation 1118, if more cycles are needed, the process1100 returns to operation 1106. If no more cycles are needed, theprocess 1100 continues to operation 1120.

In operation 1120, a coupling layer is deposited. The coupling layer 212can be a thin layer, for example less than 25 percent the thickness ofthe switching layer. The coupling layer 212 can be deposited using anydeposition method, such as ALD, PVD, etc. In operation 1122, the topelectrode (e.g. the electrode 208) is deposited.

In operation 1124, the memory element is annealed. The annealing canremove unreacted precursor ligands that may exist in the film because ofthe low deposition temperature of the ALD process. In one example, theelement is annealed using a hydrogen/argon mixture (e.g. 2-10% hydrogen,90-98% argon), although other anneals such as vacuum anneals, oxidizinganneals, etc. can be used.

FIG. 12 is a flowchart describing a process 1400 for forming a memoryelement according to various embodiments. The process 1200 can be usedin conjunction with the process described in operation 1100 if sodesired.

In operation 1202, a bottom electrode is deposited on a substrate. Thebottom electrode can be, for example, n-type polysilicon or othermaterials described herein. In operation 1204, a switching layer 210 isdeposited on the bottom electrode. The switching layer 210 can be any ofthe materials described above, for example, HfOx, CuOx, ZnOx, CeOx, etc.The switching layer 210 can be deposited using appropriate depositiontechniques such as ALD or PEALD.

In operation 1206, the switching layer 210 is optionally doped. Forexample, the switching layer 210 can be doped to create a p-typeswitching layer (e.g. a p-type ZnO or CuO layer). In operation 1208 acoupling layer, such as a metallic (e.g. Ti) or metal oxide (e.g. ZrO₂,Al₂O₃) is deposited on the switching layer.

In operation 1210, the memory element is annealed, for example using arapid thermal oxidation (RTO), a hydrogen/argon mixture, a vacuumanneal, etc. The annealing can cause oxygen to migrate from the metaloxide switching layer 210 to the coupling layer 212 in some embodiments(see e.g. FIG. 2B).

Although the foregoing examples have been described in some detail forpurposes of clarity of understanding, the invention is not limited tothe details provided. There are many alternative ways of implementingthe invention. The disclosed examples are illustrative and notrestrictive.

What is claimed is:
 1. A resistive-switching memory element comprising:a first electrode and a second electrode; a switching layer between thefirst electrode and the second electrode, wherein the switching layercomprises hafnium oxide and has a first thickness, wherein the switchinglayer comprises a first portion and a second portion, wherein the firstportion of the switching layer comprises an oxygen deficient metaloxide, wherein the second portion of the switching layer comprises ametal oxide, wherein an oxygen concentration in the second portion ofthe switching layer is greater than an oxygen concentration in the firstportion of the switching layer; and a coupling layer between theswitching layer and the second electrode, wherein the coupling layercomprises a first portion and a second portion, wherein the firstportion of the coupling layer comprises substantially titanium metal,wherein the second portion of the coupling layer comprises substantiallytitanium oxide, and wherein the first portion of the switching layerdirectly interfaces the second portion of the coupling layer.
 2. Thememory element of claim 1, wherein the first electrode is doped siliconand the memory element is configured to receive a negative reset voltagerelative to a common electrical reference and a positive set voltagerelative to the common electrical reference at the second electrode. 3.The memory element of claim 1, wherein the first thickness is between 20and 100 angstroms.
 4. The memory element of claim 1, wherein the oxygendeficient metal oxide of the first portion of the switching layercomprises a hafnium oxide having an elemental composition of betweenHfO_(1.2) and HfO_(1.7).
 5. The memory element of claim 4, wherein thefirst electrode is n-type polysilicon.
 6. The memory element of claim 1,further comprising an interface layer between the first electrode andthe switching layer, wherein the interface layer has a thickness lessthan 10 Å.
 7. The memory element of claim 1, wherein the oxygendeficient metal oxide of the first portion of the switching layercomprises a hafnium oxide having an oxygen concentration that is between60 and 95% of stoichiometric.
 8. The memory element of claim 1, wherein:the first electrode is selected from the group consisting of dopedsilicon and titanium nitride; and the second electrode is selected fromthe group consisting of molybdenum nitride, molybdenum oxide, titaniumnitride, tungsten, tantalum nitride, molybdenum nitride, molybdenumoxide, platinum, ruthenium, nickel, iridium, iridium oxide, atitanium/aluminum alloy, and ruthenium oxide.
 9. The memory element ofclaim 1, wherein the first electrode comprises doped silicon and whereinthe memory element further comprises an interface layer between thefirst electrode and the switching layer, wherein the interface layercomprises silicon oxide and has a thickness of less than 10 Å.
 10. Aresistive-switching memory element comprising: a first electrodecomprising doped silicon; a second electrode comprising a materialselected from the group consisting of a transition metal, a transitionmetal alloy, a transition metal nitride, and a transition metal carbide;a switching layer between the first electrode and the second electrode,wherein the switching layer comprises a first portion and a secondportion, wherein the first portion of the switching layer comprises anoxygen deficient metal oxide, wherein the second portion of theswitching layer comprises a metal oxide, wherein an oxygen concentrationin the second portion of the switching layer is greater than an oxygenconcentration in the first portion of the switching layer, wherein atleast one of the oxygen deficient metal oxide and the metal oxidecomprises a metal oxide selected from a group consisting of hafniumoxide, zirconium oxide, titanium oxide, cerium oxide, zinc oxide, andcopper oxide; and a coupling layer between the resistive-switching metaloxide and the second electrode, wherein the coupling layer comprises afirst portion and a second portion, wherein the first portion of thecoupling layer comprises substantially titanium metal, wherein thesecond portion of the coupling layer comprises substantially titaniumoxide, and wherein the first portion of the switching layer directlyinterfaces the second portion of the coupling layer.
 11. Aresistive-switching memory element comprising: a first electrode and asecond electrode; a switching layer disposed between the first andsecond electrode; and a coupling layer disposed between the switchinglayer and the second electrode; and wherein the switching layercomprises a first portion and a second portion disposed between thefirst portion and the first electrode; and wherein the first portion ofthe switching layer comprises a first density of oxygen vacancies;wherein the second portion of the switching layer comprises a seconddensity of oxygen vacancies; wherein the first density is higher thanthe second density; wherein the coupling layer comprises a first portionand a second portion, wherein the first portion of the coupling layercomprises substantially the titanium metal, wherein the second portionof the coupling layer comprises substantially titanium oxide, andwherein the first portion of the switching layer directly interfaces thesecond portion of the coupling layer.
 12. The memory element of claim11, further comprising an interface layer disposed between the switchinglayer and the first electrode.
 13. The memory element of claim 12,wherein the switching layer further comprises a third portion disposedbetween the second portion and the interface layer, wherein the thirdportion has a density of oxygen deficiencies that is higher than thesecond portion.
 14. The memory element of claim 12, wherein theinterface layer comprises silicon oxide.
 15. The memory element of claim11, wherein the a second electrode comprises a material selected fromthe group consisting of a transition metal, a transition metal alloy, atransition metal nitride, and a transition metal carbide.
 16. The memoryelement of claim 11, wherein the switching layer comprises a metal oxideselected from the group consisting of hafnium oxide, zirconium oxide,titanium oxide, cerium oxide, zinc oxide, and copper oxide.